1. Field of the Invention
This invention relates to surgical devices generally and more specifically to orthopedic bone plates suitable for internally fixating and stabilizing fractured bones.
2. Description of the Related Art
Many bony fractures require stabilization that cannot be provided by external splints or casts; internal fixation is therefore required. Bone plates are among the most common artificial orthopedic implants, and are commonly used to stabilize and internally fixate bony fractures.
A conventional bone plate is essentially a rigid metal plate drilled with guide holes through which bone screws can be passed. Bone screws are usually inserted through the mounting holes and threaded into the bone above and below the fracture to fix the bone plate, thereby stabilizing and fixating the fracture. Often the bone plate is removed after healing has occurred (although not necessarily).
More recently, physicians have given increasing emphasis on bone plates and devices which are capable of providing compression of the fracture as well as stabilization. Most conventional compression plates are made of metal materials having modulus much higher than that of bone. Use of such plates produces a mechanical system in which the majority of the stress is borne by the plate rather than the bone, a situation sometime referred to as “stress-shielding.” This situation is deleterious even to healthy, uncompromised bone, and can seriously impair the healing process in a fractured bone. Furthermore, it is now known that a controlled compressive load should be maintained across a fracture to promote rapid healing. Conventional, static bone plates do not provide or maintain such conditions.
Some bone plates include provision for introducing compression across the fracture when setting the plate. Usually the method of producing compression relies on an unusual bone screw or an unusual relationship between the screw and the mounting holes. Such methods can introduce initial compression, but the compression is difficult to maintain. Small movements of the bone interact with the typically high-modulus metallic plate, causing large fluctuations of the compressive load. Furthermore, some resorption may occur as a prelude to osteosynthetic growth, resulting in contraction of the bone in the region of the fracture. Even small contractions will produce slack sufficient to leave the fracture without compression (because the high-modulus metal plate cannot accommodate the contraction).
Alternatives to metal materials have been explored by some, including bioabsorbable materials and synthetic composite materials. Such materials appear promising, but offer their own challenges. There are still unanswered questions concerning the biocompatibility, strength, stability, reliability, wear, and ease of manufacture and handling. Most physicians continue to prefer metal plates to synthetic, for these reasons.